We begin tonight with that crises boiling over in egypt. Tonight crowds are once more amassing in cairo's tahrir square, the country's first freely elected president deposed and sits under house arrest, a new interim leader sworn in. Tension is splintering, jup lags on one side, on the other side anger among millions of followers of the muslim brotherhood. The u.S. National security team spent this holiday in the situation room monitoring this important american all lie. The big question, is egypt on the brink of civil war. Alex marquardt has the latest tonight from cairo. Reporter: Good evening, elizabeth. A huge military crackdown is underway here tonight. The crowds here in tahrir square are celebrating the arrest of egypt's first democratic president in 5,000 years. But top aides from his party, the muslim brotherhood, are behind bars. Tonight, egypt is a tinderbox, and morsi's supporters are warning of bloody violence to come. A heart drawn in the sky -- a message from egypt's military to their supporters on the square below. I'm so, so, so happy. I'm so happy. I'm so proud at the same time. Reporter:24 hours after the military deposed a president elected by the people, sources tell abc news he is being held with his family in a presidential guesthouse. Top officials from the muslim brotherhood -- morsi's party -- have also been rounded up. 300 arrest warrants have been issued for their members. Sunday since, clashes between the supporters and opponents have left around 50 people dead. Today, the military installed a new virtually unknown president, egypt's top constitutional judge. Do you know the name of the new president? No Reporter: The past few days have highlighted the army's historic and tremendous power, the strongest institution in the country. Egypt's presidents have all been military men. Its first civilian president, morsi, lasted just a year. In egypt, the military has far more than just the traditional role. They control a huge part of the economy, with estimates up to 40%. They build roads and bridges and have their own brands of olive oil and water. At the core of the u.S.-Egypt friendship are the country's militaries. Egypt receives $1.3 billion a year from the u.S. Washington is hoping that cash can help avoid an all-out civil war. Today, defense secretary hagel called this man -- egypt's military chief general al-sissi. For now egypt is looking to the past, a strong man in control of the country, not the new president but the head of the army. The muslim brotherhood and its allies are calling for protests tomorrow against the military. They're calling it the friday of rejection. They say they want to keep it peaceful but things are so intention here in cairo that anything could set it off. As alex just said, one of the biggest worries is if and how the millions strong muslim brotherhood and supporters will retail yat. Reporter: If the egyptian army thought the muslim brotherhood would take defeat quietly they were mistaken. We walked the streets of cairo today and they are ripe for bloodshed. The muslim brotherhood is far fr from conceiting defeat. We had to go through sever tiers of security. At each o you see piles of rocks. My life, my body. I will fight with my body. Morsi must -- you don't have to understand arabic to understand his meaning. Tonight in cairo for the defeated this is street democracy. Here supporters don't talk about how president mohammed morsi miss used his power, only how he lost it. If that's not a military coup, what's a military coup? Reporter: With much of its leadership in jail or hiding, they're inexperienced but eager. What will they do? Members we talked with would only say they're weighing their options. They have the will to fight in the streets. The question is will they? Byron pitts, abc news, cairo. As we said here at home the u.S. Is keeping a close eye on this key american ally. Each year the u.S. Sends 1.6 billion dollars in aid, second only to israel. Tonight president obama is calling for a review of that aid. Jonathan karl joins us from washington. How concerned are they about a possible civil war. Reporter: It is the top concern at the white house tonight. That is why on this holiday for the second night in a row you have seen the president huddling with his national security advisers in the situation room. Egypt is the key player in the middle east, in the arab world, the top economic power, military power, political power. So while the white house says it's deeply concerned with the way morsi have been overthrown, you have already seen the administration in contact with the top military leaders and they have stopped short of calling this a coup. They are also calling for elections as quickly as possible. Haven't the events of the past day rendered elections moot. If you don't like what he's doing, who is going to step up and run again? It's a great question but the bottom line for the white house even if they wouldn't exactly put it this way is when it comes to egypt stability is more important than democracy. They will work with this new military team. They will talk about the need to eventually get to a democratic process, but they can not see a civil war break out in the most important arab country in the middle east. Turning now to the 4th of

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